Twitter should position itself as the defacto platform for broadcast micro-blogging/messaging, and establish universal access standards for public use. They're executing on the strategy with integration together with both Google Friend Connect and Facebook Connect, allowing Google/Facebook users easy to access to Twitter and offering content distribution channels for their broadcasts.
Twitter's Open API platform has spawned a feast of applications that port into the Twitter stream. It's unlike any application - one is usually directly on Facebook, LinkedIn or a browser add-on to interact with the application. Twitter can be accessed through third party clients like Tweetdeck or Twhirl, or through value-add porting applications like Twitblogs which gives user enhanced blog-editing features.
The problem with such portability is the risk of Twitter account hijacking. I divulge my Twitter password to each third party application, and change the password often. There have been calls for solutions - i.e., master password and guest password, but the elegant feature of GFC and FB are its automated logon through a PC's cookies instead of an online password exchange. This will make Twitter a broad population application.